The invention concerns a servo regulation circuit (servo control loop) for an optical scanning means for focusing a light beam onto a recording medium or for guiding the light beam along the data tracks of the recording medium, which reflects the light beam onto several photodiodes; a servo signal is generated from the output signals of these (photodiodes) through subtraction (formation of differences) and this is overlaid by a high frequency oscillation due to the data recorded on the recording medium.
A CD player is equipped with, for example, two servo control loops, one focus regulation circuit and one track regulation circuit. In a CD player, which scans the recording medium, the compact disk, according to the so-called three-beam technique, the light beam of a laser is directed onto the disk by means of lenses and a prism beam splitter. An optical grating is located in the beam path so that the light beam generated by the laser is divided into the main beam and the diffracted beams.
The main beam is reflected from the disk onto a four quadrant photo detector with four photodiodes A, B, C and D, while the positive first order diffracted beam is reflected onto a photodiode E and the negative first order diffracted beam is reflected onto a photodiode F. The focusing error signal FE=(AS+BS)-(DS+CS) for the focus regulation circuit is formed from the output signals AS, BS, CS and DS of photodiodes A, B, C and D of the four quadrant photo detector. The track error signal TE=ES-FS for the track regulation circuit is generated from the output signals ES and FS of photodiodes E and F. Due to the data recorded on the disk, not only the focusing error signal but also the track error signal is overlaid by undesired high frequency components which aggravate the focus and track regulation.